Geoffrey has asked me to post this:
The accounting for missing aircraft, missing or incomplete records, the usual clerical errors, that some aircraft write offs were repaired while some sent for repair were written off all create an inevitable degree of uncertainty. Both sides could and did rotate units in and out of the battle. I am not sure there was a general trend, loss ratios tightened and loosened as tactics changed and in the statistics people would worry about the total number of fighting days, whether they provide a good enough sample given all the variables. Accuracy when describing cause of loss is a big issue.
Shorter periods require more detailed loss lists, probably the best British one is at
https://martinaviationpages.com/ Overall the British won the defensive fighting about 2 to 1, when you add Bomber and Coastal Command etc. the losses end up around 1 to 1.
For the time period 1 July to 31 October 1940, results from looking through the various older loss lists, Luftwaffe bombers seem to have shot down a minimum of 97 Spitfires and Hurricanes.
The evolution of the single seat fighter loss ratio, all causes July 108 Spitfires and Hurricanes to 57 Bf109s, 1.9 to 1 August 350 to 232, about 1.5 to 1 September, 343 to 234, about 1.5 to 1 October, 174 to 136 (removing the training unit Bf109s), 1.28 to 1.
Switching to losses on operations that were definitely or possibly due to enemy fighters the results look like
July 73 Spitfires and Hurricanes (Including 19 crashes, many related to night fighter training and 1 unknown), to 43 Bf109 (2 crashes, 2 unknown) August 242 Spitfires and Hurricanes (8 crashes, 7 unknown), to 185
Bf109 (4 crashes, 18 unknown)
September 267 Spitfires and Hurricanes (5 crashes, 18 unknown), to 195
Bf109 (8 crashes, 3 unknown)
October 117 Spitfires and Hurricanes (24 crashes, 3 unknown), to 112
Bf109 (9 crashes, 1 unknown)
The loss ratios, including the crashes are July 1.7 to 1, August 1.3 to 1, September 1.4 to 1, October 1 to 1
The loss ratios, excluding the crashes are July 1.3 to 1, August 1.3 to 1, September 1.4 to 1, October 0.9 to 1.
Hurricane, 181 KIA, 91 MIA, 181 wounded (50 slightly), 1 PoW, from 597 aircraft losses. Dropping the slightly wounded, the results are 45.5% of losses resulted in the death of the pilot, another 22% in the pilot being wounded, so your chances of walking away basically unhurt from a Hurricane loss was around 32.5%.
Spitfire, 135 KIA, 33 MIA, 85 wounded (26 slightly), 7 PoW from 379 aircraft losses, again dropping the slightly wounded, the results are 44.3% of losses resulted in the death of the pilot, another 15.6% in the pilot being wounded, so around 41.1% of Spitfire pilots were basically unhurt when their aircraft was destroyed.
Bf109 173 KIA, 71 MIA, 82 WIA, 188 PoW, (43 of which were wounded) versus 665 Bf109 losses, so around 37% of losses resulted in the death of the pilot, another 19% resulted in the pilot being wounded, so if you were in a Bf109 that was destroyed you had a 44% chance of surviving unhurt. As far as the Luftwaffe was concerned the permanent loss rate (KIA, MIA, PoW) was 66%, plus another 19% some of whom would not recover enough to fly fighters again. So only 15% of pilots were immediately ready to fly again.
Fighter Command for July 14 to 31 had an average of 608 operational fighters with crew, in August 702, in September 687, in October 693 as noted daily figures fluctuate a lot, from a minimum of 599 on July 23 to
740 on 24 August.